Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizers and analyzers. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, and electronic games for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction. This is a common problem with most of the display modes such as Vertically Aligned (VA), Optically Compensated Bend (OCB), and Twisted Nematic (TN) mode.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or household appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCDs application as a replacement of the traditional CRT will be limited.
One of the common methods to improve the viewing angle characteristic of LCDs is to use compensation films. As is well known to those skilled in the art, the optical compensation of LCDs is often aimed at prevention of the light leakage in the dark states such as OFF state of VA and ON state of Normally-White TN and OCB modes. In these dark states, the liquid crystal optic axis inside of the liquid crystal cell takes the configuration shown schematically in FIG. 4. FIG. 4 is a cross section of a liquid crystal cell where the liquid crystal optic axis 403 changes its direction in the cell thickness direction z. Around the mid-plane 401 of the liquid crystal cell, the liquid crystal optic axis 403 is sufficiently perpendicular to the plane of the cell boundary 405. As the liquid crystal used in the liquid crystal cell for LCDs is positively birefringent, this part represents the positive C-plate. On the other hand, the optic axis 403 changes direction in the vicinity of the cell boundaries 405. It is tilted with respect to the plane of the boundary of liquid crystal cell. Thus it has a positive O-plate property. To optically compensate such a state, the compensation film has to have a similar symmetry containing C-plate and O-plate components. In order to counter the positive C part of the liquid crystal cell, the compensation film has to have a negative C element. However, as is well known in the art, a positive O component of the cell can be compensated by a negative O component, positive O component or combination of both. Thus the compensation film can have the configuration, for example, Negative C+Positive O, Negative C+Negative O, Negative C+Positive O+Positive O etc. Compensation films with these configurations have a physical structure containing optically anisotropic layers disposed on an optically transparent substrate. An O-plate layer can be made of, for example, liquid crystal polymers. As it is necessary to align the optic axis of the liquid crystal polymer in the desired direction, an alignment layer is often deposited between the optically anisotropic layer and substrate or between the two optically anisotropic layers. It is possible that the compensation films include other auxiliary layers such as adhesion promotion layers, barrier layers, and antistatic layers. Adhesion promotion layers can be put between the alignment layer and the substrate. This layer may become necessary when there is not sufficient mutual adhesion between the substrate and the alignment layer. It may be also the case that an adhesion promotion layer is placed between the substrate and O-or C-plate layers.
One often has to put barrier layer between the alignment layer and the substrate or between the layers with different chemical compositions but having the same optical properties. Typical liquid crystal alignment layers are susceptible to chemical contamination. Placing the barrier layer can prevent an undesired diffusion of compounds from the substrate to alignment layer. In order to promote static discharge, one or more antistatic layers can be placed within the compensation film. This would prevent the sticking of dust particles on the film surfaces.
As is well known in the art, methods other than deposition of liquid crystal polymer to generate O-plates are possible. This includes tilted deposition of inorganic materials and tilted holographic layers. These layers with O-plate properties are disposed on the transparent substrate, for example, triacetylcellulose (TAC), cellulose acetate butyrate (CAB), cyclic polyolefin and glass. Some polymeric substrates, such as TAC, have negative C property (thus negative Rth).
U.S. Pat. No. 5,583,679 and U.S. Pat. No. 5,805,253 disclose the compensation film with a discotic liquid crystal compound layer disposed on the substrate for TN-LCD and OCB-LCD, respectively. The discotic compound changes its tilt in optic axis with respect to the film plane, and changes its direction in the layer thickness direction; therefore a discotic liquid crystal layer is a negative O-plate. U.S. Pat. No. 5,583,679 calls for the use of a polymeric substrate such as polynorbonene type polymer and TAC. The substrate is required to have −150 nm≦Rth≦−30 nm and Rin being less than 20 nm. This property is essentially that of a negative C-plate. For OCB cell, U.S. Pat. No. 5,805,253 discloses the use of a substrate with −400 nm≦Rth≦−50 nm.
U.S. Pat. No. 5,619,352 discusses the use of Negative C combined with Positive O-plate for gray scale compensation of TN-LCD. Various combinations of A, Positive O and Negative C-plates are disclosed.
In the prior art, a need for negative C-plate with relatively large Rth is mentioned in combination with positive or negative O-plate. Some prior arts only specify the necessary value range for Rth but fail to provide the method of its generation. Others put examples of polymeric substrates. However, most of the polymeric substrates known in the art typically have |Δnth|˜1×10−3 or less. Thus in order to obtain Rth=−100 nm, the necessary substrate thickness would be 100 μm or larger. For example, the most commonly used substrate TAC provides only Rth˜−50 nm for the thickness of 80 μm. Certainly, doubling the thickness or lamination of substrates increases the negative value of Rth. This, however, also thickens the liquid crystal display unit. Therefore, it is desirable to increase the negative value of Rth of the compensation films without significant increase in the compensation film thickness. Various methods have been proposed that can be used to obtain a sufficiently large negative Rth value.
US 2001/0026338 discloses a use of retardation increasing agent in combination with TAC. The retardation-increasing agent is chosen from aromatic compounds having at least two benzene rings. The problems with this method is the amount of the doping of the agent. To generate the desired effects of increasing Rth, the necessary amount of agent is high enough to cause coloration.
As is well known in the art, cholesteric liquid crystal (CHLC) can be used to obtain Negative C-plate. The pitch of the CHLC is shorter than the wavelength of the visible light, thus properly aligned CHLC exhibits form birefringence giving negative Rth. Sasaki et al. propose (US 2003/0086033) use of short pitch CHLC disposed on thermoplastic substrate. CHLC, however, usually is costly material, and the alignment process would only increase the manufacturing cost.
Thus, it is a problem to be solved to provide a multilayer optical compensation film comprising Positive O-plate or Negative O-plate or both, and Negative C-plate with sufficiently large negative value of Rth. The Negative C-plate of said compensation film should be easily manufactured, be low cost, and should offer ease in controlling negative Rth value without significantly increasing the thickness of the compensation film so that the film is readily manufactured and provides the required optical property for proper compensation of LCDs.